A hot driver will perform better at higher levels when his is battelig or overtakin as the meter is hot but when driving alone he might get bored or something and may do mistakes as the meter is set to cold.. A hot driver will make more mistakes when he's fresh to the career
A cold driver will make more mistake when overtakin etc when the meter is hot and perform good lap times when the meter is cold e.g driving alone. A cold driver will stay cool when in the beginnin of his career but again, he'll prefer to stay behind then fighting to the top
IMO
EDIT: I've got one or two drivers set to midle of hot/cold, one or two set to cold and the rest set to hot. my hot drivers are the best drivers. L. Alonso lvl 36 hot doesnt make mistakes and gets to the top during the first two laps in remoteraces fighting with other drivers at the same level. tho fighting with a level 40 driver he'll loose.. most of the times...